Wafer processing method

ABSTRACT

A wafer processing method for forming a modified layer within a wafer along planned dividing lines forms the modified layer within the wafer, positions a condensing point within the wafer or at a top surface of the wafer and applies a second laser beam while moving the condensing point, images reflected light, and determines a processed state of the wafer on the basis of an imaged image. The second laser beam is formed such that a sectional shape of the second laser beam in a plane perpendicular to a traveling direction of the second laser beam is not axisymmetric with respect to an axis along the planned dividing lines.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to a wafer processing method that forms amodified layer as a starting point for dividing a wafer by applying alaser beam from an undersurface side of the wafer and condensing thelaser beam within the wafer, and makes a crack extend from the modifiedlayer to a top surface side of the wafer.

Description of the Related Art

In a process of manufacturing device chips, a plurality of planneddividing lines intersecting each other are set on the top surface of thewafer, a device is formed in each of demarcated regions, and the waferis divided along the planned dividing lines. For example, a laser beamhaving a wavelength transmissible through the wafer (wavelength that canpass through the wafer) is applied to the wafer from the undersurfaceside of the wafer and is condensed within the wafer along the planneddividing lines. At this time, a modified layer as a starting point fordivision is formed in the vicinity of the condensing point of the laserbeam. When a crack extends from the formed modified layer to the topsurface of the wafer, the wafer is divided along the planned dividinglines (see Japanese Patent Laid-Open No. 2005-86161 and Japanese PatentLaid-Open No. 2010-68009, for example). This processing methodnecessitates appropriate settings of processing conditions such as theformation position of the modified layer in the depth direction of thewafer, and the irradiation condition of the laser beam so that themodified layer is formed and the crack develops from the modified layerto the top surface of the wafer. When the processing conditions or thelike are not appropriate, the crack does not appropriately extend fromthe formed modified layer, or the crack extends in an unintendeddirection, for example, so that the wafer cannot be dividedappropriately. The yield of the device chips is therefore decreased. Inaddition, when the optical axis of an optical system is displaced, themodified layer is not formed at a predetermined position. Thus, again,the wafer cannot be divided appropriately.

SUMMARY OF THE INVENTION

Here, in order to check whether or not the processing conditions or thelike are appropriate, the modified layer is formed as intended, and thecrack is appropriately developed from the modified layer to the topsurface of the wafer, the top surface of the wafer may be observed by amicroscope or the like, for example. However, in order to observe thetop surface side of the wafer irradiated with the laser beam from theundersurface side, it is necessary, for example, to extract the waferfrom a laser processing apparatus, vertically invert the wafer, andcarry the wafer into a microscope or the like. Therefore, there is aproblem in that it takes man-hours to check a processed state of thewafer.

It is accordingly an object of the present invention to provide a waferprocessing method that makes it possible to check easily whether or nota wafer is appropriately processed.

In accordance with an aspect of the present invention, there is provideda wafer processing method for forming a modified layer within a waferalong a plurality of planned dividing lines of the wafer, the waferhaving the planned dividing lines set on a top surface of the wafer. Thewafer processing method includes a holding step of making the topsurface of the wafer face a chuck table, and holding the wafer by thechuck table, a modified layer forming step of forming the modified layerwithin the wafer by positioning a condensing point of a first laser beamhaving a wavelength transmissible through the wafer within the wafer andapplying the first laser beam along the planned dividing lines from anundersurface side of the wafer while moving a laser beam irradiatingunit and the chuck table relative to each other in a direction along theplanned dividing lines, an observation laser beam applying step of,after the modified layer forming step, positioning a condensing point ofa second laser beam having a power not exceeding a processing thresholdvalue of the wafer and having a wavelength transmissible through thewafer within the wafer or at the top surface of the wafer, and applyingthe second laser beam from the undersurface side of the wafer whilemoving the wafer and the condensing point relative to each other in thedirection along the planned dividing lines, an imaging step of imaging,by an imaging unit, reflected light of the second laser beam applied inthe observation laser beam applying step, and a determining step ofdetermining a processed state of the wafer on a basis of an imagedisplayed in the imaging step. The second laser beam applied to thewafer in the observation laser beam applying step is formed such that asectional shape of the second laser beam in a plane perpendicular to atraveling direction of the second laser beam is not axisymmetric withrespect to an axis along the planned dividing lines.

In addition, according to another aspect of the present invention, thereis provided a wafer processing method for forming a modified layerwithin a wafer along a plurality of planned dividing lines of the wafer,the wafer having the planned dividing lines set on a top surface of thewafer. The wafer processing method includes a holding step of making thetop surface of the wafer face a chuck table, and holding the wafer bythe chuck table, a modified layer forming step of forming the modifiedlayer within the wafer by positioning a condensing point of a firstlaser beam having a wavelength transmissible through the wafer withinthe wafer and applying the first laser beam along the planned dividinglines from an undersurface side of the wafer while moving a laser beamirradiating unit and the chuck table relative to each other in adirection along the planned dividing lines, an observation laser beamapplying step of, after the modified layer forming step, positioning acondensing point of a second laser beam having a power not exceeding aprocessing threshold value of the wafer and having a wavelengthtransmissible through the wafer within the wafer or at the top surfaceof the wafer, and applying the second laser beam from the undersurfaceside of the wafer while moving the wafer and the condensing pointrelative to each other in a direction orthogonal to the direction alongthe planned dividing lines such that the condensing point crosses themodified layer, an imaging step of imaging, by an imaging unit,reflected light of the second laser beam applied in the observationlaser beam applying step, and a determining step of determining aprocessed state of the wafer on a basis of an image photographed in theimaging step. The second laser beam applied to the wafer in theobservation laser beam applying step is formed such that a sectionalshape of the second laser beam in a plane perpendicular to a travelingdirection of the second laser beam is not axisymmetric with respect toan axis along the planned dividing lines.

Preferably, the observation laser beam applying step is performed in animmersed state.

The wafer processing method according to one aspect of the presentinvention performs the observation laser beam applying step, the imagingstep, and the determining step after performing the modified layerforming step of forming the modified layer by condensing the first laserbeam within the wafer. In the observation laser beam applying step, thesecond laser beam is applied from the undersurface side of the waferwhile the wafer and the condensing point are moved relative to eachother.

The second laser beam applied to the undersurface side of the wafer andtraveling within the wafer in the observation laser beam applying stepis reflected by the top surface of the wafer. Then, in the imaging step,an image is formed by imaging reflected light of the second laser beam.Here, the shape and position of the reflected light appearing in theimage are determined by positional relations between the modified layer,the crack, and the second laser beam, and the like. Therefore, whenimages displaying the reflected light of the second laser beam areformed while the wafer and the condensing point are moved relative toeach other, the processed state of the wafer such as the formationposition and height of the modified layer, and the presence or absenceof the crack can be determined.

Hence, one aspect of the present invention provides a wafer processingmethod that makes it possible to check easily whether or not a wafer isprocessed appropriately.

The above and other objects, features and advantages of the presentinvention and the manner of realizing them will become more apparent,and the invention itself will best be understood from a study of thefollowing description and appended claims with reference to the attacheddrawings showing a preferred embodiment of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view schematically illustrating a wafer;

FIG. 2 is a sectional view schematically illustrating a modified layerforming step;

FIG. 3A is a sectional view schematically illustrating in enlargeddimension the wafer within which a modified layer is formed;

FIG. 3B is a sectional view schematically illustrating in enlargeddimension the wafer within which a modified layer and a crack areformed;

FIG. 4 is a sectional view schematically illustrating an observationlaser beam applying step;

FIG. 5A is a sectional view schematically illustrating a second laserbeam applied to the wafer within which the modified layer is formed andreflected light of the second laser beam;

FIG. 5B is a sectional view schematically illustrating the second laserbeam applied to the wafer within which the modified layer and the crackare formed and reflected light of the second laser beam;

FIG. 6A is a plan view schematically illustrating a region irradiatedwith the second laser beam in the undersurface of the wafer;

FIG. 6B is a plan view schematically illustrating an example of a regionillustrating reflected light in an image displaying the reflected light;

FIG. 6C is a plan view schematically illustrating another example of theregion illustrating the reflected light in the image displaying thereflected light;

FIG. 7A and FIG. 7B are images displaying the reflected light in a casewhere the crack is formed in the wafer;

FIG. 7C and FIG. 7D are images displaying the reflected light in a casewhere the crack is not formed in the wafer;

FIG. 8A is a plan view schematically illustrating regions irradiatedwith the second laser beam;

FIG. 8B is a sectional view schematically illustrating paths of thesecond laser beam and the reflected light;

FIG. 9A, FIG. 9B, FIG. 9C, FIG. 9D, and FIG. 9E are plan viewsschematically illustrating regions illustrating the reflected light inimages displaying the reflected light;

FIG. 10A is a plan view schematically illustrating regions irradiatedwith the second laser beam;

FIG. 10B is a sectional view schematically illustrating paths of thesecond laser beam and the reflected light;

FIG. 11A, FIG. 11B, FIG. 11C, FIG. 11D, and FIG. 11E are plan viewsschematically illustrating regions illustrating the reflected light inimages displaying the reflected light; and

FIG. 12 is a flowchart illustrating a flow of steps of a waferprocessing method.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

An embodiment of the present invention will be described with referenceto the accompanying drawings. Description will first be made of a waferin which a modified layer is formed by a wafer processing methodaccording to the present embodiment. FIG. 1 is a perspective viewschematically illustrating a wafer 1. The wafer 1 is, for example, asubstantially disk-shaped substrate or the like formed of a materialsuch as silicon (Si), silicon carbide (SiC), gallium nitride (GaN),gallium arsenide (GaAs), or another semiconductor, or a material such assapphire, glass, or quartz. The glass is, for example, alkali glass,non-alkali glass, soda-lime glass, lead glass, borosilicate glass,quartz glass, or the like. A plurality of planned dividing lines 3intersecting each other are set on a top surface 1 a of the wafer 1. Theplanned dividing lines 3 are referred to also as streets. On the topsurface 1 a of the wafer 1, a device 5 is formed in each of regionsdemarcated by the planned dividing lines 3. The device 5 is, forexample, an integrated circuit (IC), a large-scale integrated circuit(LSI), or the like. However, the wafer 1 is not limited to this. Thereare no limitations on the material, shape, structure, size, and the likeof the wafer 1, and the device 5 may not be formed on the wafer 1.

When the wafer 1 is divided along the planned dividing lines 3,individual device chips having respective devices 5 are formed. At atime of dividing the wafer 1, for example, a modified layer is formedwithin the wafer 1 by condensing a laser beam within the wafer 1 alongthe planned dividing lines 3, and a crack is formed which extends alonga thickness direction from the modified layer to the top surface 1 a ofthe wafer 1. At this time, unless the processing conditions of the wafer1 are appropriate and also the state of a laser processing apparatus isa state suitable for processing, the crack does not extend from themodified layer, or the crack extends in an unintended direction, forexample, so that the wafer 1 cannot be divided appropriately. In thiscase, defective products are produced, and thus the yield of devicechips is decreased.

Next, referring to FIG. 2 and the like, description will be made of alaser processing apparatus 2 that performs a method of processing thewafer 1 according to the present embodiment. FIG. 2 is a sectional viewschematically illustrating a state in which a modified layer is formedin the wafer 1 by using the laser processing apparatus 2. The laserprocessing apparatus 2 includes a chuck table 4 that holds the wafer 1and a laser beam irradiating unit 6 that irradiates the wafer 1 held onthe chuck table 4 with a laser beam. The chuck table 4 has a porousmember (not illustrated) on an upper surface side. The upper surface ofthe porous member forms a holding surface 4 a on which the wafer 1 isheld. The chuck table 4 is rotatable about an axis perpendicular to theholding surface 4 a. The chuck table 4 has a suction source (notillustrated) connected to the porous member.

At a time of processing the wafer 1 by the laser processing apparatus 2,the wafer 1 is placed on the holding surface 4 a with the top surface 1a facing the holding surface 4 a, and then a negative pressure producedby the suction source is made to act on the wafer 1 through the porousmember. In this case, the wafer 1 is sucked and held on the chuck table4 in a state in which an undersurface 1 b side is exposed upward. Thewafer 1 is laser-processed by being irradiated with the laser beam fromthe exposed undersurface 1 b side.

At a time of holding the wafer 1 on the chuck table 4, a frame unit maybe formed in advance which is obtained by integrating an annular frame,an adhesive tape whose periphery is affixed to the annular frame, andthe wafer 1 with each other. At a time of forming the frame unit, thetop surface 1 a side of the wafer 1 is affixed to an adhesive surface ofthe adhesive tape exposed in an opening of the annular frame. In thiscase, at a time of holding the frame unit on the chuck table 4, thewafer 1 is placed on the holding surface 4 a via the adhesive tape.

The chuck table 4 and the laser beam irradiating unit 6 can be movedrelative to each other in a direction parallel with the holding surface4 a. For example, the chuck table 4 can be moved in a processing feeddirection (X-axis direction) set as a direction parallel with theholding surface 4 a, and the laser beam irradiating unit 6 can be movedin an indexing feed direction (Y-axis direction) parallel with theholding surface 4 a and orthogonal to the processing feed direction.

FIG. 2 schematically illustrates a simplest configuration example of thelaser beam irradiating unit 6 that can irradiate the wafer 1 held on thechuck table 4 with a laser beam. The laser beam irradiating unit 6includes a laser oscillator 8 for oscillating a laser, a mirror 10, anda condensing lens 12.

The laser oscillator 8 has a function of emitting a first laser beam 14of a wavelength transmissible through the wafer 1 (wavelength thatpasses through the wafer 1). A laser having a wavelength of 1099 nm andoscillated with Nd:YAG or the like as a medium, for example, is used asthe first laser beam 14. However, the laser oscillator 8 and the firstlaser beam 14 are not limited to this but are selected according to thematerial of the wafer 1 or the like. At a time of forming a modifiedlayer within the wafer 1, the power of the first laser beam 14 is set toapproximately 2 to 3 W, for example. However, the power of the firstlaser beam 14 is not limited to this, but it suffices for the power ofthe first laser beam 14 to be a power at which a modified layer can beformed within the wafer 1. The first laser beam 14 emitted from thelaser oscillator 8 is reflected in a predetermined direction by themirror 10 and is applied to the wafer 1 held on the chuck table 4through the condensing lens 12.

The condensing lens 12 has a function of condensing the first laser beam14 at a predetermined height position within the wafer 1 held on thechuck table 4. The condensing lens 12 is, for example, movable along aheight direction and can thereby change the height position of acondensing point 16. The condensing point 16 of the first laser beam 14is positioned at a predetermined height position within the wafer 1. Asillustrated in FIG. 2, a modified layer 7 is formed within the wafer 1when the first laser beam 14 is condensed within the wafer 1 while thelaser beam irradiating unit 6 and the chuck table 4 are moved relativeto each other along the processing feed direction. Here, when processingconditions such as the irradiation condition of the first laser beam 14,and a processing feed speed are set appropriately, a crack 9 extendingfrom the modified layer 7 to the top surface la of the wafer 1 is formedas illustrated in FIG. 3B, so that the wafer 1 can be divided easily andappropriately.

However, when the processing conditions or the like are inappropriate,the crack 9 does not extend appropriately from the formed modified layer7, as illustrated in FIG. 3A, or the crack 9 extends in an unintendeddirection, for example, so that the wafer 1 cannot be dividedappropriately. Further, when the optical axis of an optical system isdisplaced, the modified layer 7 is not formed at a predeterminedposition, so that the wafer 1 cannot be divided appropriately. The yieldof the device chips is therefore decreased. Here, in order to checkwhether or not the wafer 1 is processed appropriately, the top surface 1a of the wafer 1 may, for example, be observed by a microscope or thelike. That is, the top surface 1 a of the wafer 1 may be observed by amicroscope or the like to check whether or not the crack 9 is developedappropriately from the modified layer 7 to the top surface 1 a of thewafer 1 and whether or not the modified layer 7 is formed appropriatelyat an intended position.

However, in order to observe the top surface la side of the wafer 1irradiated with the first laser beam 14 from the undersurface 1 b side,it is necessary, for example, to extract the wafer 1 from the laserprocessing apparatus 2, vertically invert the wafer 1, and carry thewafer 1 into a microscope or the like. Therefore, there is a problem inthat it takes man-hours to check a processed state such as the presenceor absence of the crack 9. Accordingly, the wafer processing methodaccording to the present embodiment reduces the checking man-hours bychecking the processed state of the wafer 1 in the laser processingapparatus 2. A configuration used to check the processed state will nextbe described.

As illustrated in FIG. 4, the laser processing apparatus 2 includes anobservation laser beam irradiating unit 18. The observation laser beamirradiating unit 18 has a function of irradiating the wafer 1 in whichthe modified layer 7 is formed with a second laser beam 28 as a laserbeam for observation. FIG. 4 schematically illustrates a simplestconfiguration example of the observation laser beam irradiating unit 18that can irradiate the wafer 1 held on the chuck table 4 with the secondlaser beam 28. The observation laser beam irradiating unit 18 includes alaser oscillator 20, a dichroic mirror 22, a condensing lens 24, and abeam forming unit 26 that forms the shape of the second laser beam 28into a specific shape. The laser oscillator 20 can emit the second laserbeam 28 with a power not exceeding a processing threshold value at whicha modified layer can be formed within the wafer 1.

The laser oscillator 20, for example, emits the second laser beam 28with a power of approximately 0.2 W not exceeding the processingthreshold value. However, the power of the second laser beam 28 is notlimited to this. The processing threshold value differs according to thematerial of the wafer 1. Thus, the power of the second laser beam 28 isdetermined appropriately so as not to exceed the processing thresholdvalue according to the material of the processed wafer 1. Preferably,the power of the second laser beam 28 is set between one tenth and onethousandth of the power of the first laser beam 14. More preferably, thepower of the second laser beam 28 is set at approximately one thirtiethof the first laser beam 14.

The dichroic mirror 22 has a function of reflecting the second laserbeam 28 in a predetermined direction. The dichroic mirror 22 also has afunction of transmitting reflected light 32 of the second laser beam 28when the reflected light 32 reaches the dichroic mirror 22 after thesecond laser beam 28 is reflected by the top surface 1 a side of thewafer 1, as will be described later. The condensing lens 24 has afunction of condensing the second laser beam 28 within the wafer 1 heldon the chuck table 4 or at the top surface 1 a of the wafer 1. Thecondensing lens 24 is, for example, movable along the height direction,and can thereby change the height position of a condensing point 30.

Incidentally, the observation laser beam irradiating unit 18 may be ableto irradiate the wafer 1 held on the chuck table 4 with the first laserbeam 14 having a power exceeding the processing threshold value of thewafer 1. That is, the observation laser beam irradiating unit 18 may beable to function as the laser beam irradiating unit 6 described withreference to FIG. 2. In this case, the laser beam irradiating unit 6 canbe omitted, so that the configuration of the laser processing apparatus2 is simplified. Hence, the first laser beam 14 and the second laserbeam 28 may be from a same light source. On the other hand, the wafer 1can be processed efficiently when the laser processing apparatus 2 hasanother chuck table in the case where the laser processing apparatus 2includes both of the laser beam irradiating unit 6 and the observationlaser beam irradiating unit 18. For example, it is possible to irradiateone wafer 1 with the second laser beam 28 and simultaneously irradiateanother wafer 1 with the first laser beam 14.

The beam forming unit 26 included in the observation laser beamirradiating unit 18 has a function of forming the shape of the secondlaser beam 28 emitted from the laser oscillator 20 into a specificshape. The beam forming unit 26 is, for example, a plate-shaped memberhaving a transmitting window (not illustrated) having a shapecorresponding to the specific shape and a shielding portion (notillustrated) that shields the second laser beam 28 on the periphery ofthe transmitting window. The transmitting window is formed so as topenetrate the beam forming unit 26. The beam forming unit 26 isincorporated into the observation laser beam irradiating unit 18 whilethe orientation of the beam forming unit 26 is adjusted such that thepenetrating direction of the transmitting window coincides with thetraveling direction of the second laser beam 28. When the second laserbeam 28 reaches the beam forming unit 26, the second laser beam 28 isformed into the specific shape with a part of the second laser beam 28passing through the transmitting window and with the rest shielded bythe shielding portion.

Alternatively, a diffractive optical element (DOE) may be incorporatedas the beam forming unit 26 in the observation laser beam irradiatingunit 18. In this case, the DOE is designed and manufactured so as to beable to form the second laser beam 28 into a predetermined shape.Further, a spatial light modulator including a liquid crystal on silicon(LCOS) element may be incorporated as the beam forming unit 26 in theobservation laser beam irradiating unit 18.

In the wafer processing method according to the present embodiment, thesecond laser beam 28 is formed such that the sectional shape of thesecond laser beam 28 in a plane (for example, the undersurface 1 b)perpendicular to the traveling direction of the second laser beam 28 isnot axisymmetric with respect to an axis assumed along a planneddividing line 3 when the second laser beam 28 is applied to theundersurface 1 b of the wafer 1. For example, the sectional shape of thesecond laser beam 28 is a semicircular shape positioned on one side oftwo regions separated from each other by the axis. The second laser beam28 is applied from the undersurface 1 b side to the wafer 1, and travelswithin the wafer 1. Then, reaching the top surface 1 a of the wafer 1,the second laser beam 28 is reflected by the top surface 1 a of thewafer 1. Thereafter, the reflected light 32 of the second laser beam 28travels in an opposite direction within the wafer 1, and travels fromthe undersurface 1 b to the outside of the wafer 1.

The reflected light 32 of the second laser beam 28 is converted intocollimated light by passing through the condensing lens 24 and passesthrough the dichroic mirror 22. An imaging unit 34 that images thereflected light 32 is disposed on the traveling path of the reflectedlight 32 passing through the dichroic mirror 22. The imaging unit 34,for example, includes an image sensor such as a complementary metaloxide semiconductor (CMOS) sensor or a charge coupled device (CCD)sensor. The imaging unit 34 images the reflected light 32 and forms animage displaying the reflected light 32. As will be described later,determination of the processed state of the wafer 1 as to whether or notthe crack 9 extends appropriately from the modified layer 7 formedwithin the wafer 1 to the top surface 1 a or the like is made on thebasis of the image formed by the imaging unit 34 by imaging thereflected light 32. In addition, the image is used also when whether ornot the modified layer 7 is appropriately formed at an intended positionis determined by detecting the formation position of the modified layer7, for example.

The wafer processing method according to the present embodiment willnext be described. The wafer processing method is, for example,performed in the laser processing apparatus 2. The wafer processingmethod forms the modified layer 7 within the wafer 1 along the planneddividing lines 3 of the wafer 1 having the plurality of planned dividinglines 3 set on the top surface 1 a. FIG. 12 is a flowchart of assistancein explaining a flow of steps of the wafer processing method. In thefollowing, each of the steps will be described in detail.

First, a holding step S10 is performed which carries the wafer 1 intothe laser processing apparatus 2, makes the top surface 1 a of the wafer1 face the chuck table 4, and holds the wafer 1 by the chuck table 4. Inthe holding step S10, the top surface 1 a side of the wafer 1 is made toface the holding surface 4 a of the chuck table 4 such that theundersurface 1 b side of the wafer 1 is exposed upward, and the wafer 1is placed on the chuck table 4. Thereafter, when a negative pressure ismade to act on the wafer 1 by actuating the suction source of the chucktable 4, the wafer 1 is sucked and held on the chuck table 4. FIG. 2schematically illustrates a sectional view of the wafer 1 sucked andheld on the chuck table 4. Incidentally, before the holding step S10 isperformed, a protective member disposing step may be performed withaffixes a protective member such as an adhesive tape to the top surface1 a of the wafer 1 in advance. In this case, the wafer 1 is held on thechuck table 4 via the protective member in the holding step S10.

Next, a modified layer forming step S20 is performed which formsmodified layers 7 within the wafer 1 by applying the first laser beam 14along the planned dividing lines 3 from the undersurface 1 b side of thewafer 1. The first laser beam 14 is a laser beam of a wavelengthtransmissible through the wafer 1 (wavelength that can pass through thewafer 1). FIG. 2 is a sectional view schematically illustrating themodified layer forming step S20. In the modified layer forming step S20,first, the chuck table 4 and the laser beam irradiating unit 6 are movedrelative to each other, and one end of one planned dividing line 3 ofthe wafer 1 is positioned below the laser beam irradiating unit 6. Atthe same time, the planned dividing line 3 of the wafer 1 is alignedwith the processing feed direction by rotating the chuck table 4. Then,the condensing point 16 of the first laser beam 14 is positioned at apredetermined height position within the wafer 1.

Thereafter, the wafer 1 is irradiated with the first laser beam 14 whilethe chuck table 4 and the laser beam irradiating unit 6 are movedrelative to each other in the processing feed direction. When the wafer1 is irradiated with the first laser beam 14 under conditionsappropriate for the processing of the wafer 1, a modified layer 7 alongthe planned dividing line 3 is formed within the wafer 1, and a crack 9(see FIG. 3B and the like) extending from the modified layer 7 to thetop surface 1 a of the wafer 1 is formed. After the modified layer 7 isformed along the one planned dividing line 3 of the wafer 1, the chucktable 4 and the laser beam irradiating unit 6 are moved relative to eachother in the indexing feed direction, and a modified layer 7 issimilarly formed within the wafer 1 along another planned dividing line3. After modified layers 7 are formed along all of the planned dividinglines 3 along one direction, the chuck table 4 is rotated, and amodified layer 7 is similarly formed along planned dividing lines 3along another direction. The modified layer forming step S20 iscompleted when the first laser beam 14 is applied along all of theplanned dividing lines 3 of the wafer 1. Incidentally, in each of theplanned dividing lines 3, a plurality of modified layers 7 superposed oneach other may be formed by applying the first laser beam 14 twice ormore while changing the height of the condensing point 16.

When the wafer 1 within which the modified layer 7 and the crack 9extending from the modified layer 7 are formed along each planneddividing line 3 is thinned by grinding the wafer 1 from the undersurface1 b side, and the modified layer 7 and the like are removed, the wafer 1is divided, and individual device chips are obtained. However, the wafer1 cannot be divided appropriately unless the crack 9 appropriatelyextends to the top surface 1 a of the wafer 1. In addition, the wafer 1cannot be divided appropriately also in a case where the formationposition of the modified layer 7 is not at an intended position or in acase where the modified layer 7 meanders without forming a linear shapealong the planned dividing line 3. In these cases, the quality of theformed device chips may not meet a standard. In addition, the devicechips may be damaged. That is, the yield of the device chips isdecreased.

FIG. 3A is a sectional view schematically illustrating in enlargeddimension the wafer 1 within which the modified layer 7 is formed butthe crack 9 is not formed. In addition, FIG. 3B is a sectional viewschematically illustrating in enlarged dimension the wafer 1 withinwhich the crack 9 reaching the top surface la from the modified layer 7is formed together with the modified layer 7. As illustrated in FIG. 3B,when the crack 9 reaches the top surface la, and the top surface la ofthe wafer 1 is observed by a microscope, the crack 9 is visuallyrecognized. When the crack 9 is not formed, on the other hand, the crack9 cannot be visually recognized on the top surface 1 a. Accordingly, inorder to check the presence or absence of the crack 9 after the modifiedlayer 7 is formed in the wafer 1, the top surface 1 a side of the wafer1 may be observed by a microscope. However, in order to observe the topsurface la by a microscope, the wafer 1 needs to be carried out from thechuck table 4 and moved to the microscope. Accordingly, in order todetermine the processed state of the wafer 1, the wafer processingmethod according to the present embodiment performs an observation laserbeam applying step S30, an imaging step S40, and a determining step S50.

Next, description will be made of the observation laser beam applyingstep S30 performed after the modified layer forming step S20. In theobservation laser beam applying step S30, the wafer 1 held on the chucktable 4 is irradiated with the second laser beam 28 as an observationlaser beam from the observation laser beam irradiating unit 18. Thesecond laser beam 28 is a laser beam having a power not exceeding theprocessing threshold value of the wafer 1 and having a wavelengthtransmissible through the wafer 1 (wavelength that can pass through thewafer 1). FIG. 4 is a side view schematically illustrating theobservation laser beam applying step S30. At a time of irradiating thewafer 1 having the modified layer 7 formed therewithin with the secondlaser beam 28 from the undersurface 1 b side, the condensing point 30 ispositioned within the wafer 1 or at the top surface 1 a of the wafer 1in advance. Preferably, the condensing point 30 is positioned at aposition coinciding with the planned dividing line 3 on the top surface1 a of the wafer 1 or in the vicinity of the position.

The second laser beam 28 emitted from the laser oscillator 20 reachesthe beam forming unit 26 and is formed into a predetermined shape by thebeam forming unit 26. Thereafter, the second laser beam 28 is reflectedby the dichroic mirror 22, and travels toward the chuck table 4. Then,after passing through the condensing lens 24, the second laser beam 28is applied to the undersurface 1 b of the wafer 1, travels within thewafer 1, and is condensed to the condensing point 30. The second laserbeam 28 traveling within the wafer 1 is reflected by the top surface 1 aof the wafer 1. Then, the reflected light 32 of the second laser beam 28travels within the wafer 1, and travels to the outside through theundersurface 1 b of the wafer 1. The reflected light 32 thereafterpasses through the condensing lens 24 and the dichroic mirror 22 andreaches the imaging unit 34.

FIG. 6A is a plan view schematically illustrating an example of thesectional shape of the second laser beam 28 applied to the wafer 1.Specifically, FIG. 6A illustrates a region 40 irradiated with the secondlaser beam 28 in the undersurface 1 b of the wafer 1. The region 40 ishatched. Further, for the convenience of description, FIG. 6Aillustrates a broken line schematically representing the plane positionof the modified layer 7 formed within the wafer 1 along the planneddividing line 3 and a point schematically representing the planeposition of the condensing point 30. As illustrated in FIG. 6A, thesectional shape of the second laser beam 28 is, for example, asemicircular shape. As illustrated in FIG. 6A, the second laser beam 28is formed by the beam forming unit 26 in advance such that the sectionalshape of the second laser beam 28 in a surface (for example, theundersurface 1 b of the wafer 1) perpendicular to the travelingdirection of the second laser beam 28 is asymmetric with respect to anaxis assumed along the planned dividing line 3.

Here, detailed description will be made of the path of the reflectedlight 32 of the second laser beam 28 reflected at the condensing point30 located at the top surface 1 a of the wafer 1. FIG. 5A is a sectionalview schematically illustrating the traveling paths of the second laserbeam 28 and the reflected light 32 in a case where the crack 9 extendingfrom the modified layer 7 to the top surface 1 a of the wafer 1 is notformed. FIG. 5B is a sectional view schematically illustrating the pathsof the second laser beam 28 and the reflected light 32 in a case wherethe crack 9 extending from the modified layer 7 is formed. Incidentally,the sectional view of FIG. 5A and the sectional view of FIG. 5B aredrawings of assistance in explaining effects of the presence or absenceof the crack 9 on the reflected light 32. For the convenience ofdescription, the sectional view of FIG. 5A and the sectional view ofFIG. 5B emphasize characteristics such as relative positional relationsbetween the wafer 1, the modified layer 7, the planned dividing line 3,and the crack 9, and angles at which the second laser beam 28 and thereflected light 32 travel.

As illustrated in FIG. 5A and FIG. 5B, the second laser beam 28 appliedto the undersurface 1 b side of the wafer 1 is condensed to thecondensing point 30. Then, the second laser beam 28 is reflected by thetop surface la of the wafer 1, and the reflected light 32 travels withinthe wafer 1 and reaches the undersurface 1 b of the wafer 1. In a casewhere the condensing point 30 is located below the modified layer 7 andin the top surface la, when the crack 9 reaching the top surface 1 a ofthe wafer 1 from the modified layer 7 is not formed within the wafer 1,the second laser beam 28 passes and travels through a region below themodified layer 7. As illustrated in FIG. 5A, the second laser beam 28(incident light) and the reflected light 32 are inverted from each otherwith the modified layer 7 interposed therebetween.

In the case where the crack 9 reaching the top surface 1 a of the wafer1 from the modified layer 7 is formed within the wafer 1, on the otherhand, the second laser beam 28 reaches the crack 9 below the modifiedlayer 7 and is affected by the crack 9. In the case where the crack 9reaches the top surface 1 a of the wafer 1, the wafer 1 is slightlydivided by the crack 9, and therefore an interface is formed between alayer of air entering the crack 9 and the wafer 1. Because light isreflected at the interface with a large difference between refractiveindexes on both sides, the second laser beam 28 reaching the crack 9 isreflected by the crack 9 as in the case where the second laser beam 28is reflected at the top surface la. In this case, as illustrated in FIG.5B, the reflected light 32 travels backward in a region similar to aregion within the wafer 1 through which region the second laser beam 28(incident light) has passed, and the reflected light 32 reaches theundersurface 1 b of the wafer 1.

Further, in the observation laser beam applying step S30, the chucktable 4 and the observation laser beam irradiating unit 18 are movedrelative to each other along the planned dividing line 3. For example,the chuck table 4 is moved in the processing feed direction (X-axisdirection). Specifically, the undersurface 1 b of the wafer 1 issuccessively irradiated with the second laser beam 28 while the wafer 1and the condensing point 30 are moved relative to each other along theplanned dividing line 3. In a case where the modified layer 7 formed inthe modified layer forming step S20 is linearly formed along the planneddividing line 3 without meandering, each time the undersurface 1 b ofthe wafer 1 is irradiated with the second laser beam 28, the secondlaser beam 28 is similarly reflected at the top surface la. As a result,the reflected light 32 travels within the wafer 1 in a similar path. Onthe other hand, in a case where the processing conditions of the wafer 1are inappropriate and the modified layer 7 meanders, the traveling pathof the reflected light 32 changes when a meandering part of the modifiedlayer 7 is irradiated with the second laser beam 28. It is thereforepossible to evaluate the processed state of the wafer 1 by repeatedlyobserving the reflected light 32.

Alternatively, in the observation laser beam applying step S30, theobservation laser beam irradiating unit 18 is moved in the indexing feeddirection (Y-axis direction), for example. Specifically, theundersurface lb of the wafer 1 is successively irradiated with thesecond laser beam 28 while the wafer 1 and the condensing point 30 aremoved in a direction orthogonal to the direction along the planneddividing line 3 such that the condensing point 30 crosses the modifiedlayer 7 (such that the condensing point 30 passes below the modifiedlayer 7). There is, for example, a case where the processing conditionsare not appropriate in the modified layer forming step S20, the modifiedlayer 7 is consequently formed at a height position that is too distantfrom the top surface la, and as a result, the crack 9 extending from themodified layer 7 is not formed. In this case, in order to obtain aknowledge of a formation depth of the modified layer 7, the wafer 1 isrepeatedly irradiated with the second laser beam 28 while the condensingpoint 30 is moved so as to cross the modified layer 7. At this time, thesecond laser beam 28 or the reflected light 32 hits the modified layer7. When the condensing point 30 moves, a manner in which the secondlaser beam 28 or the like hits the modified layer 7 changes, and thusthe traveling path of the reflected light 32 changes. The manner of thischange depends on the formation height of the modified layer 7. Theprocessed state of the wafer 1 can therefore be evaluated by repeatedlyobserving the reflected light 32.

The wafer processing method according to the present embodiment nextperforms the imaging step S40 of imaging, by the imaging unit 34, thereflected light 32 of the second laser beam 28 applied to the wafer 1 inthe observation laser beam applying step S30. In the imaging step S40,the reflected light 32 is imaged, and an image displaying the reflectedlight 32 is formed.

FIG. 6C is a plan view schematically illustrating a region 42 billustrating the reflected light 32 in an image 38 imaged and formed bythe imaging unit 34 in the case where the crack 9 reaching the topsurface 1 a from the modified layer 7 is not formed. In the case wherethe crack 9 is not formed, as illustrated in FIG. 5A, the second laserbeam 28 (incident light) and the reflected light 32 are inverted fromeach other with the modified layer 7 interposed therebetween. Therefore,the shape of the reflected light 32 of the second laser beam 28appearing in the image 38 is a shape such that the sectional shape ofthe second laser beam 28 is inverted. In a case where the sectionalshape of the second laser beam 28 is a semicircular shape, the region 42b illustrating the reflected light 32 has a shape obtained by invertingthe semicircular shape, as illustrated in FIG. 6C.

In addition, FIG. 6B is a plan view schematically illustrating a region42 a illustrating the reflected light 32 in an image 36 imaged andformed by the imaging unit 34 in the case where the crack 9 extends fromthe modified layer 7 to the top surface la. In the case where the crack9 extends from the modified layer 7 to the top surface 1 a, the paths ofthe second laser beam 28 (incident light) and the reflected light 32coincide with each other, as illustrated in FIG. 5B. Therefore, in thecase in which the sectional shape of the second laser beam 28 is asemicircular shape, the region 42 a illustrating the reflected light 32has a shape similar to the semicircular shape, as illustrated in FIG.6B.

Thus, the shape or the like of the reflected light 32 appearing in theimages 36 and 38 formed by imaging the reflected light 32 in the imagingstep S40 changes according to the presence or absence of the crack 9. Itis therefore possible to determine whether or not the crack 9 reachingthe top surface 1 a from the modified layer 7 is formed within the wafer1 on the basis of the images 36 and 38.

FIG. 7A and FIG. 7B are photographs illustrating an example of imagesphotographed by the imaging unit 34 in the case where the crack 9 isformed in the wafer 1. In addition, FIG. 7C and FIG. 7D are photographsillustrating an example of images photographed by the imaging unit 34 inthe case where the crack 9 is not formed in the wafer 1. Each of thephotographs illustrates, in white, the reflected light 32 of the secondlaser beam 28 reflected at the top surface 1 a of the wafer 1. The shapeand position of the reflected light 32 appearing in the images changeaccording to whether or not the crack 9 is formed in the wafer 1. It istherefore understood that the shape and position of the reflected light32 appearing in each photograph can be a criterion for determining theprocessed state of the wafer 1, the processed state being typified bythe presence or absence of the crack 9 or the like. Incidentally, as isunderstood from each photograph, the reflected light 32 does notnecessarily appear with uniform intensity in the region illustrating thereflected light 32. That is, the reflected light 32 is not necessarilydistributed uniformly in the entire region 42 a illustrated in FIG. 6Bor in the entire region 42 b illustrated in FIG. 6C. The reflected light32 appears in a stripe shape or a spot shape in the images due tovarious factors caused by an optical phenomenon and the like. However,the processed state of the wafer 1 can be determined adequately evenwhen the reflected light 32 appears nonuniformly in the images.

The wafer processing method according to the present embodiment performsthe determining step S50 of determining the processed state of the wafer1 on the basis of images photographed in the imaging step S40. Here, theprocessed state, for example, refers to the state of the wafer 1processed by applying the first laser beam 14, and includes a processingresult. The processed state, for example, refers to the presence orabsence of the crack 9 reaching the top surface 1 a from the modifiedlayer 7, a height position at which the modified layer 7 is formed, thepresence or absence of the meandering of the modified layer 7, and thelike. Details of determination made in the determining step S50 will bedescribed. In the determining step S50, the processed state of the wafer1 is determined from the position and shape of the reflected light 32appearing in images photographed in the imaging step S40. Descriptionwill first be made of the determining step S50 in a case where the wafer1 is irradiated with the second laser beam 28 while the condensing point30 is relatively moved in a direction along a planned dividing line(X-axis direction) in the observation laser beam applying step S30.

FIG. 8A is a plan view schematically illustrating a region irradiatedwith the second laser beam 28 in the undersurface 1 b of the wafer 1 inthe observation laser beam applying step S30. The modified layer 7 alongthe planned dividing line 3 is formed within the wafer 1 illustrated inFIG. 8A. The formation position of the modified layer 7 is representedby a broken line in FIG. 8A. In the wafer 1 illustrated in FIG. 8A, acrack reaching the top surface 1 a from the modified layer 7 is formed,but the modified layer 7 is meandered.

In the example illustrated in FIG. 8A, the condensing point 30 ispositioned at a position superposed on the modified layer 7 in the topsurface la, and the undersurface 1 b of the wafer 1 is irradiated withthe second laser beam 28 five times while the condensing point 30 isrelatively moved along the planned dividing line 3. In FIG. 8A, regions44 a, 44 b, 44 c, 44 d, and 44 e irradiated with the second laser beam28 are hatched to represent the position of the second laser beam 28 inthe undersurface 1 b. FIGS. 9A to 9E are plan views schematicallyillustrating images formed by imaging the reflected light 32 in theimaging step S40. For example, FIG. 9A is a plan view schematicallyillustrating an image illustrating the reflected light 32 imaged by theimaging unit 34 when the irradiation region 44 a in FIG. 8A isirradiated with the second laser beam 28. Similarly, figures from FIG.9B to FIG. 9E are respectively plan views schematically illustratingimages displaying the reflected light 32 imaged by the imaging unit 34when the irradiation regions 44 b, 44 c, 44 d, and 44 e are irradiatedwith the second laser beam 28.

The traveling paths of the second laser beam 28 and the reflected light32 traveling within the wafer 1 when the irradiation region 44 a isirradiated with the second laser beam 28 are similar to the travelingpaths illustrated in FIG. 5B. That is, the second laser beam 28 isreflected by the top surface 1 a of the wafer 1 and the crack 9, and thereflected light 32 travels backward in the traveling path of the secondlaser beam 28 and travels from the undersurface 1 b to the outside ofthe wafer 1. Therefore, in an image 46 a illustrated in FIG. 9A, thereflected light 32 appears in a region 48 a having a shape similar tothe sectional shape of the second laser beam 28.

FIG. 8B illustrates the traveling paths of the second laser beam 28 andreflected light 32 a and 32 b traveling within the wafer 1 when theirradiation region 44 b is irradiated with the second laser beam 28. Inthis case, a part of the second laser beam 28 traveling within the wafer1 reaches the condensing point 30 and is reflected by the top surface 1a of the wafer 1. The reflected light 32 a travels from the undersurface1 b of the wafer 1 to the outside and reaches the imaging unit 34. Then,in an image 46 b schematically illustrated in a plan view of FIG. 9B,the reflected light 32 a appears in a region 48 b. Meanwhile, asillustrated in FIG. 8B, another part of the second laser beam 28traveling within the wafer 1 reaches the modified layer 7 and the crack9, is reflected by the modified layer 7 and the crack 9 and is furtherreflected by the top surface 1 a at a position that is not thecondensing point 30. Thereafter, the reflected light 32 b travels fromthe undersurface 1 b of the wafer 1 to the outside and reaches theimaging unit 34. Then, in the image 46 b schematically illustrated inthe plan view of FIG. 9B, the reflected light 32 b appears in a region50 b.

Similarly, in an image 46 c schematically illustrated in FIG. 9C, thereflected light not affected by the modified layer 7 appears in a region48 c, and the reflected light affected by the modified layer 7 appearsin a region 50 c. In an image 46 d schematically illustrated in FIG. 9D,the reflected light not affected by the modified layer 7 appears in aregion 48 d. In an image 46 e schematically illustrated in FIG. 9E, thereflected light of a part of the second laser beam 28 reflected at thecondensing point 30 is reflected by the modified layer 7 and appears ina region 50e, and remaining reflected light appears in a region 48ewithout hitting the modified layer 7.

It is to be noted that the changes appearing in the regions illustratingthe reflected light 32 according to the processed state of the wafer 1in the images obtained by imaging the reflected light 32 in the imagingstep S40 are not limited to this. That is, changes appear in variousmodes in the regions illustrating the reflected light in the imagesaccording to the thickness of the wafer 1 and the formation depth of themodified layer 7, the position of each constituent element of theobservation laser beam irradiating unit 18, and the like. At any rate,however, the positions and shapes of the regions illustrating thereflected light in the images are determined by the processed state ofthe wafer 1. It is therefore possible to determine the processed stateof the wafer 1 from the images obtained in the imaging step S40.

On the basis of the images 46 a and 46 d, for example, it is suggestedthat the modified layer 7 is formed at an intended position in theirradiation regions 44 a and 44 d irradiated with the second laser beam28. On the other hand, changes in the reflected light appearing in theimages 46 b, 46 c, and 46 e are observed, and it is suggested that themodified layer 7 is formed so as to be separated from the intendedposition in the irradiation regions 44 b, 44 c, and 44 e. It cantherefore be determined in the determining step S50 that the modifiedlayer 7 is partially meandered as the processed state of the wafer 1.

Next, description will be made of the determining step S50 in a casewhere the wafer 1 is irradiated with the second laser beam 28 while thewafer 1 and the condensing point 30 are relatively moved in a directionorthogonal to the direction along the planned dividing line (Y-axisdirection) such that the condensing point 30 crosses the modified layer7.

FIG. 10A is a plan view schematically illustrating regions irradiatedwith the second laser beam 28 in the undersurface 1 b of the wafer 1 inthe observation laser beam applying step S30. The modified layer 7 alongthe planned dividing line 3 is formed within the wafer 1 illustrated inFIG. 10A. The formation position of the modified layer 7 is representedby a broken line in FIG. 10A. In the wafer 1 illustrated in FIG. 10A,however, the crack reaching the top surface 1 a from the modified layer7 is not formed.

In the example illustrated in FIG. 10A, the condensing point 30 ispositioned at a position shifted in a Y-axis direction from the modifiedlayer 7 in the top surface 1 a, and the undersurface 1 b of the wafer 1is irradiated with the second laser beam 28 five times while thecondensing point 30 is relatively moved along the Y-axis direction so asto cross the modified layer 7. In FIG. 10A, regions 52 a, 52 b, 52 c, 52d, and 52 e irradiated with the second laser beam 28 are hatched torepresent the position of the second laser beam 28 in the undersurface 1b. FIGS. 11A to 11E are plan views schematically illustrating imagesformed by imaging the reflected light 32 in the imaging step S40.

For example, FIG. 11A is a plan view schematically illustrating an imagedisplaying the reflected light 32 which image is formed by the imagingunit 34 when the irradiation region 52 a in FIG. 10A is irradiated withthe second laser beam 28. Similarly, figures from FIG. 11B to 11E arerespectively plan views schematically illustrating images displaying thereflected light 32 which images are formed by the imaging unit 34 whenthe irradiation regions 52 b, 52 c, 52 d, and 52 e are irradiated withthe second laser beam 28.

Because the irradiation region 52 a is greatly separated from themodified layer 7, the second laser beam 28 and the reflected light 32traveling within the wafer 1 do not hit the modified layer 7. That is,the second laser beam 28 is reflected by the top surface la of the wafer1, and the reflected light 32 travels to the outside of the wafer 1.Therefore, in an image 54 a illustrated in FIG. 11A, the reflected light32 appears in a region 56 a having a shape obtained by inverting thesectional shape of the second laser beam 28. FIG. 10B schematicallyillustrates the traveling paths of the second laser beam 28 andreflected light 32 c and 32 d traveling within the wafer 1 when theirradiation region 52 b is irradiated with the second laser beam 28. Inthis case, the second laser beam 28 traveling within the wafer 1 reachesthe condensing point 30 and is reflected by the top surface 1 a. Then, apart of the reflected light 32 c travels from the undersurface 1 b ofthe wafer 1 to the outside and reaches the imaging unit 34. Then, in animage 54 b schematically illustrated in a plan view of FIG. 11B, thereflected light 32 c appears in a region 56 b. Meanwhile, as illustratedin FIG. 10B, another part of the reflected light 32 d reaches themodified layer 7, is reflected by the modified layer 7, travels from theundersurface 1 b of the wafer 1 to the outside, and reaches the imagingunit 34. Then, in the image 54 b schematically illustrated in the planview of FIG. 11B, the reflected light 32 d appears in a region 58 b.

When the irradiation region 52 c is irradiated with the second laserbeam 28, the second laser beam 28 and the reflected light 32 travel in atraveling path similar to the traveling path illustrated in FIG. 5A.Therefore, in an image 54 c schematically illustrated in FIG. 11C, thereflected light 32 appears in a region 56 c having a shape obtained byinverting the sectional shape of the second laser beam 28.

When the irradiation region 52 d is irradiated with the second laserbeam 28, a part of the second laser beam 28 reaches the modified layer7, is reflected by the modified layer 7, and is reflected by the topsurface la at a position that is not the condensing point 30. In animage 54 d schematically illustrated in FIG. 11D, the reflected lightreflected by the top surface 1 a at the position that is not thecondensing point 30 appears in a region 58 d. Further, another part ofthe second laser beam 28 reaches the condensing point 30 without hittingthe modified layer 7, and is reflected by the top surface 1 a, and thereflected light appears in a region 56 d in an image 54.

When the irradiation region 52 e is irradiated with the second laserbeam 28, the second laser beam 28 and the reflected light 32 travelingwithin the wafer 1 do not hit the modified layer 7. That is, the secondlaser beam 28 is reflected by the top surface 1 a of the wafer 1, andthe reflected light 32 travels to the outside of the wafer 1. Therefore,in an image 54 e illustrated in FIG. 11E, the reflected light 32 appearsin a region 56 e having a shape obtained by inverting the sectionalshape of the second laser beam 28.

Thus, whether or not the second laser beam 28 or the reflected light 32hits the modified layer 7 can be determined on the basis of therespective images illustrated in FIGS. 11A to 11E. The presence orabsence of reflection of the second laser beam 28 or the like by themodified layer 7 changes while the condensing point 30 is moved so as tocross the modified layer 7. It is therefore possible to derive theformation height of the modified layer 7 from information such as theposition of the condensing point 30 during a period during which thereflection of the second laser beam 28 or the like by the modified layer7 is detected, the shape of the second laser beam 28, and thearrangement of the optical system included in the observation laser beamirradiating unit 18. Hence, in the determining step S50, the formationheight of the modified layer 7 can be determined as the processed stateof the wafer 1. Further, the quality of the modified layer 7 such as astate of unevenness of the reflecting surface of the modified layer 7 asa reflecting surface may be determined from the distribution of thereflected light 32 in an image displaying the reflected light 32.

In addition, the position of the modified layer 7 in the Y-axisdirection can also be determined precisely on the basis of each image.When the position of the modified layer 7 can be determined precisely,the second laser beam 28 can be applied with the condensing point 30positioned so as to be superposed on the modified layer 7. For example,when the second laser beam 28 is applied with the condensing point 30precisely positioned at a position superposed on the modified layer 7 inthe top surface 1 a of the wafer 1, and an image displaying thereflected light 32 is formed, the presence or absence of the crack 9extending from the modified layer 7 to the top surface 1 a can bedetermined with high accuracy (see FIG. 5A and FIG. 5B). As describedabove, in the determining step S50, the processed state of the wafer 1is determined on the basis of the images displaying the reflected light32 of the second laser beam 28.

Here, the observation laser beam applying step S30, the imaging stepS40, and the determining step S50 may be performed repeatedly. Forexample, first, the observation laser beam applying step S30 isperformed while the condensing point 30 is moved along the Y-axisdirection so as to cross the modified layer 7. Then, the position of themodified layer 7 in the Y-axis direction can be identified in thedetermining step S50. Thereafter, the condensing point 30 is positionedso as to be superposed on the modified layer 7, and the observationlaser beam applying step S30 is performed while the condensing point 30is moved along the X-axis direction. Then, whether or not the modifiedlayer 7 is meandered can be determined in the determining step S50.

It is to be noted that the shape and position of the reflected light 32appearing in the images obtained in the imaging step S40 are not limitedto this. For example, in the case where the crack 9 is not formed in thewafer 1, the reflected light 32 displayed in the images may not have theshape obtained by inverting the sectional shape of the incident lightdepending on the condensing position of the second laser beam 28 and thedisposition position of the imaging unit 34. That is, effects of theprocessed state of the wafer 1 on the position and shape of thereflected light 32 appearing in the images differ according to eachsystem. Accordingly, the effects are preferably verified in advance inthe case where the processed state of the wafer 1, the processed statebeing typified by the presence or absence of the crack 9 extending fromthe modified layer 7 to the top surface 1 a, is to be determined fromthe images obtained in the imaging step S40.

For example, a wafer 1 in which the crack 9 is formed in advance and awafer 1 in which the crack 9 is not formed are prepared, each of thewafers 1 is irradiated with the second laser beam 28, and images areobtained by similarly imaging the reflected light 32. In addition, awafer 1 in which the formation position and formation depth of themodified layer 7 are known is prepared, the wafer 1 is irradiated withthe second laser beam 28 while the condensing point 30 is variouslymoved with respect to the modified layer 7, and images are obtained bysimilarly imaging the reflected light 32. Then, effects of the presenceor absence of the crack 9 and the formation depth and formation positionof the modified layer 7 on the images and the like are evaluatedmultilaterally. That is, it is preferable to evaluate the effects of theprocessed state of the wafer 1 on the images and create a criterion fordetermining the processed state of the wafer 1 from the images.

Incidentally, when the modified layer 7 is formed in the wafer 1, themodified layer 7 in a linear shape may be formed at a position separatedfrom a center line of the planned dividing line 3 even in a case wherethe modified layer 7 is formed without meandering. A distance betweenthe formation position of the modified layer 7 and the center line ofthe planned dividing line 3 is referred to as a kerf displacement. Thewafer processing method according to the present embodiment may evaluatean amount of kerf displacement by determining the formation position ofthe modified layer 7. In addition, when the modified layer 7 is formedin the wafer 1, a crack 9 not reaching the top surface 1 a may extendfrom the modified layer 7. When wafers 1 in which the length of thecrack 9 extending from the modified layer 7 is known are prepared, andimages displaying the reflected light 32 are similarly formed, arelation between the length of the crack 9 and the positions and shapesof regions illustrating the reflected light 32 in the images may beobtained. In this case, in the determining step S50, the length of thecrack 9 can be calculated from the images displaying the reflected light32.

Further, the wafer processing method according to the present embodimentobtains information about a position at which a processing abnormalityis caused in the wafer 1 by irradiating the undersurface 1 b side of thewafer 1 with the second laser beam 28 while moving the condensing point30 and obtaining a plurality of images displaying the reflected light32. Accordingly, the wafer processing method according to the presentembodiment may store the information about the position at which theprocessing abnormality is caused in a control unit of the laserprocessing apparatus 2 or the like. In this case, for example, on thebasis of the information about the position at which the processingabnormality is caused, device chips obtained from the position at whichthe processing abnormality is caused can be identified as defectiveproducts among the individual device chips obtained by dividing thewafer 1. In addition, an operator of the laser processing apparatus 2can adjust or repair constituent elements of the laser processingapparatus 2 on the basis of the information about the position at whichthe processing abnormality is caused in the wafer 1.

As described above, in the wafer processing method according to thepresent embodiment, the presence or absence of the crack 9 can be easilydetermined on the spot without the wafer 1 being moved from the chucktable 4 of the laser processing apparatus 2. That is, the processedstate of the wafer 1 can be determined easily.

It is to be noted that the present invention is not limited to thedescription of the foregoing embodiment but can be modified and carriedout in various manners. For example, while in the foregoing embodiment,description has been made mainly of a case where the wafer 1 isirradiated with the first laser beam 14 and the second laser beam 28from the undersurface 1 b side, one aspect of the present invention isnot limited to this. For example, the first laser beam 14 and the secondlaser beam 28 may be applied to the top surface 1 a side of the wafer 1.In addition, a wafer 1 on which the devices 5 are not formed may belaser-processed to form the modified layer 7 within the wafer 1. Inaddition, while in the foregoing embodiment, description has been madeby taking as an example a case where the sectional shape of the secondlaser beam 28 is a semicircular shape, the sectional shape is notlimited to this. For example, the sectional shape may be a triangularshape, a quadrangular shape, or another polygonal shape. That is, itsuffices for the distribution of power to be asymmetric with respect tothe axis along the planned dividing line 3 (for example, the modifiedlayer 7).

Incidentally, there is a case where the second laser beam 28 applied tothe wafer 1 is not precisely condensed to the condensing point 16 due toan effect of spherical aberration, and consequently the reflected light32 is not clearly displayed in the images obtained in the imaging stepS40. Accordingly, the condensing lens 24 may be fitted with a correctionring that alleviates the effect of spherical aberration. In this case,for example, a correction ring having appropriate performance accordingto the thickness and material of the wafer 1 is selected and used.Alternatively, in a case where a spatial light modulator such as an LCOSelement is used in the observation laser beam irradiating unit 18, thesecond laser beam 28 corrected for spherical aberration may be formedand applied to the undersurface 1 b of the wafer 1.

Further, the observation laser beam applying step S30 may be performedin an immersed state. Describing the observation laser beam applyingstep S30 illustrated in FIG. 4 in this case, a space between thecondensing lens 24 and the undersurface 1 b of the wafer 1 is filledwith a liquid. A liquid referred to as an immersion oil, glycerin, orpure water, for example, can be used as the liquid. In a case where theobservation laser beam applying step S30 is performed in an immersedstate, the numerical aperture of the condensing lens 24 functioning asan objective lens can be increased. Therefore, the resolution of imagesdisplaying the reflected light 32 imaged by the imaging unit 34 can beenhanced, so that the processed state of the wafer 1 can be analyzed inmore detail.

The present invention is not limited to the details of the abovedescribed preferred embodiment. The scope of the invention is defined bythe appended claims and all changes and modifications as fall within theequivalence of the scope of the claims are therefore to be embraced bythe invention.

What is claimed is:
 1. A wafer processing method for forming a modifiedlayer within a wafer along a plurality of planned dividing lines of thewafer, the wafer having the planned dividing lines set on a top surfaceof the wafer, the wafer processing method comprising: a holding step ofmaking the top surface of the wafer face a chuck table, and holding thewafer by the chuck table; a modified layer forming step of forming themodified layer within the wafer by positioning a condensing point of afirst laser beam having a wavelength transmissible through the waferwithin the wafer and applying the first laser beam along the planneddividing lines from an undersurface side of the wafer while moving alaser beam irradiating unit and the chuck table relative to each otherin a direction along the planned dividing lines; an observation laserbeam applying step of, after the modified layer forming step,positioning a condensing point of a second laser beam having a power notexceeding a processing threshold value of the wafer and having awavelength transmissible through the wafer within the wafer or at thetop surface of the wafer, and applying the second laser beam from theundersurface side of the wafer while moving the wafer and the condensingpoint relative to each other in the direction along the planned dividinglines; an imaging step of imaging, by an imaging unit, reflected lightof the second laser beam applied in the observation laser beam applyingstep; and a determining step of determining a processed state of thewafer on a basis of an image photographed in the imaging step, whereinthe second laser beam applied to the wafer in the observation laser beamapplying step being formed such that a sectional shape of the secondlaser beam in a plane perpendicular to a traveling direction of thesecond laser beam is not axisymmetric with respect to an axis along theplanned dividing lines.
 2. A wafer processing method for forming amodified layer within a wafer along a plurality of planned dividinglines of the wafer, the wafer having the planned dividing lines set on atop surface of the wafer, the wafer processing method comprising: aholding step of making the top surface of the wafer face a chuck table,and holding the wafer by the chuck table; a modified layer forming stepof forming the modified layer within the wafer by positioning acondensing point of a first laser beam having a wavelength transmissiblethrough the wafer within the wafer and applying the first laser beamalong the planned dividing lines from an undersurface side of the waferwhile moving a laser beam irradiating unit and the chuck table relativeto each other in a direction along the planned dividing lines; anobservation laser beam applying step of, after the modified layerforming step, positioning a condensing point of a second laser beamhaving a power not exceeding a processing threshold value of the waferand having a wavelength transmissible through the wafer within the waferor at the top surface of the wafer, and applying the second laser beamfrom the undersurface side of the wafer while moving the wafer and thecondensing point relative to each other in a direction orthogonal to thedirection along the planned dividing lines such that the condensingpoint crosses the modified layer; an imaging step of imaging, by animaging unit, reflected light of the second laser beam applied in theobservation laser beam applying step; and a determining step ofdetermining a processed state of the wafer on a basis of an imagephotographed in the imaging step, wherein the second laser beam appliedto the wafer in the observation laser beam applying step being formedsuch that a sectional shape of the second laser beam in a planeperpendicular to a traveling direction of the second laser beam is notaxisymmetric with respect to an axis along the planned dividing lines.3. The wafer processing method according to claim 1, wherein theobservation laser beam applying step is performed in an immersed state.4. The wafer processing method according to claim 2, wherein theobservation laser beam applying step is performed in an immersed state.